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Intelligent Power Switches Slide 15

Fast demagnetization can bring a lot of heat also, and the energy stored in the inductive load must be dissipated very efficiently. As an example, using an IPS that has a nominal current of 0.5 A at 25 V (which is common for VCC), without taking into account the voltage drop across a MOS and a diode, when the IPS is ON, the voltage on the output pin is 25 V and the inductive load is energized. When the IPS is turned off, the current decreases and a negative voltage across the load appears. The IPS integrates a zener diode between the OUT and the VCC that will clamp the voltage at 50 V. In other words, when the transistor is turned OFF, the voltage between VCC and OUT can go up to 50 V. So the designer ends up with a VDS voltage of the transistor at around 50 V, that means up to 25 W. Thanks to a chopping technique, STMicro can handle this situation. With the thermal resistance junction to ambient of 50 C/W, if no action was performed, that would mean an increase of 1250°C, in other words, the system would be destroyed.

PTM Published on: 2011-09-07